Asbestos-induced chromosomal abnormalities have been studied in human pleural mesothelial cells and in bronchial fibroblast cultures in vitro. After one exposure to amosite, a significant increase in chromosomal aberrations was found in mesothelial cells, as compared to unexposed controls. The number of aberrant cells was further increased in cultures treated twice wth asbestos, the major aberration type being dicentric chromosomes and acentric fragments (minute chromosomes). The number of aneuploid cells was observed to increase in all the mesothelial cell cultures with the passage number of the culture. However, no significant differences were found between the asbestos-exposed and control mesothelial cells. In asbestos-exposed fibroblasts, on the other hand, the number of aneuploid cells was significantly elevated, as compared to unexposed controls.